To increase the efficiency of gas turbine engines, a known approach is to raise the turbine operating temperatures. As operating temperatures are increased, the thermal limits of certain engine components may be exceeded, resulting in material failures or, at the very least, reduced service life. In addition, the increased thermal expansion and contraction of these components adversely effects clearances and their interfitting relationships with other components of different thermal coefficients of expansion. Consequently, these components must either be cooled or limited in their exposure to the high temperature working gas to avoid potentially damaging consequences at elevated operating temperatures. It is common practice to extract from the main airstream a portion of the compressed air at the output of the compressor for cooling purposes. So as not to unduly compromise the gain in engine operating efficiency achieved through higher operating temperatures, the amount of extracted cooling air should be held to a small percentage of the total main airstream. This requires that the cooling air be utilized with utmost efficiency in maintaining the temperatures of these components within safe limits.
A particularly critical component subjected to extremely high temperatures is the shroud located immediately downstream from the high pressure turbine nozzle from the combustor. The shroud closely surrounds the rotor of the high pressure turbine and thus defines the outer boundary for the extremely high temperature, energized working gas stream flowing through the high pressure turbine. To prevent material failure and to maintain proper clearance with the rotor blades of the high pressure turbine, adequate shroud cooling is a critical concern.
High pressure turbine shrouds are typically formed as a circumferential array of arcuate shroud segments. Gaps are provided between adjacent shroud segments to accommodate differential thermal expansion of the shroud segments and their supporting structure. As these axially and radially extending gaps are exposed to the working gas stream on their radially inner sides and typically cooling air on their radially outer sides, they must be sealed. The gap seals should be of a character to minimize the leakages of working gas and cooling air radially through the gaps and also accommodate variations in the gap width due to thermal expansion and contraction.
There are numerous examples of shroud seals in the prior art that are effect we in minimizing radial leakages of working gas and cooling air. Unfortunately, these conventional seals are not effective in limiting the axial flow of working gas in the gaps. That is, the gaps are typically open at their fore (upstream) and aft (downstream) ends, and, consequently, working gas enters the fore ends of the gaps, flows axially in the gaps due to pressure differential and exits their aft ends. The edges of the shroud segments defining the inter-segment gaps are thus heated by the working gas to extremely high temperatures damaging to the shroud material integrity. These gaps must therefore be cooled. To this end, U.S. Pat. Nos. 4,650,394 and 4,767,260 propose using perforated gap seals to accommodate a metered flow of high pressure cooling air radially through the gap for cooling the shroud segment edges, as well as disrupting the axial flow of working gas in the gaps. This approach is inefficient, as it requires a significant quantity of cooling air to achieve the intended purpose.
It is accordingly an object of the present invention to provide an improved shroud seal in gas turbine engines.
A further object is to provide a shroud seal of the above-character which minimizes the leakages of hot working gas and cooling air radially through the gaps between segments of the shroud in the high pressure turbine section of a gas turbine engine.
An additional object is to provide a seal of the above-character, wherein the axial flow of hot working gas in the shroud segment gaps is minimized.
Another object is to provide a seal of the above-character, which accommodates efficient cooling of the shroud edges defining the inter-segment gaps.
A still further object is to provide a seal of the above-character, which accommodates variations in the inter-segment gap width due to thermal expansion and contraction.
Other objects of the invention will in part be obvious and in part appear hereinafter.